The present invention relates to a curable composition for paint which has low toxicity and high solid content, and improved acid resistance, scratch resistance, storage stability as a one package and curability. More particularly, the invention relates to a curable composition useful as a paint for top coatings of automobiles.
Hitherto, the resins for paint which have been used for coating automobiles, outer walls of buildings, industrial equipments, stem furnitures, plastics and the like, are mainly melamine resins such as an alkydmelamine and an acryl-melamine or two component urethane resins.
Among them, the melamine resins involve the problems that harmful formalin arises in curing and that the cured films are spoiled with acid rain because of poor acid resistance. In the case of the two component urethane resins, the toxicity of the isocyanate compound used in a manufacturing process becomes a problem.
In order to solve such problems, the present inventors had studied the use of a vinyl polymer having a silicon atom to which a hydrolyzable group is bonded (hereinafter also referred to as "hydrolyzable silyl group") for paint. As a result, it was found that the use of the above-mentioned vinyl polymer solved the above-mentioned problems and the resin was a curable paint resin having an excellent weatherability, as described in the previously filed patent applications (refer to Japanese Unexamined Patent Publication No. 132977/1988 and the like).
The above-mentioned vinyl polymer having a hydrolyzable silyl group is cured according to the following mechanism. Namely, the silyl group therein is hydrolyzed by moisture in air, stable siloxane bonds are formed through condensation reaction, and then the vinyl polymer is cured. Therefore, the above-mentioned vinyl polymer has excellent chemical resistance and weatherability in comparison with the melamine resins such as acryl-melamine and alkyd-melamine or the two component urethane resins.
However, when the above-mentioned vinyl polymer having a hydrolyzable silyl group is used as a paint for top coatings of automobiles, the following problems arise.
Firstly, scratch resistance to a brush for car washing and cloud of sand caused in running is insufficient. When a flexible component such as a polyester component is copolymerized, or blended with the above-mentioned vinyl monomer in order to improve the scratch resistance, the acid resistance of the resulting film is deteriorated.
Secondly, in recent years a high solid paint is required, because there arises the movement to control the total amount of solvent which volatilizes during the paint coating, as the VOC (volatile organic compounds) control in U.S.A. However, in the case of a paint consisting of only the above-mentioned polymer it is difficult to obtain a high solid content paint satisfactory for the VOC control in U.S.A.
Thirdly, when a tin compound such as di-n-butyl tin (IV) dilaurate, a sulfonic acid such as p-toluenesulfonic acid, or a phosphoric ester including an acid phosphate such as di( 2-ethylhexyl)phosphate is added as a catalyst, one package storage stability is not always sufficient due to their high activities even at ordinary temperature.
An object of the invention is to provide a curable composition for paint having excellent high solid content and one package storage stability as well as superior scratch resistance and acid resistance.
This and other objects of the present invention will become apparent from the description hereinafter.